Biofilms are microbial communities encased in a matrix of extracellular polymeric substance composed of extracellular DNA, proteins, lipids, and polysaccharides which adhere to and grow on biotic and abiotic surfaces. Biofilm formation is a genetically controlled process in the life cycle of bacteria that produces numerous changes in the cellular physiology of the organism, often including increased antibiotic resistance (of up to 100 to 1000 times), as compared to growth under planktonic (free floating) conditions. Diverse microorganisms form biofilms in response to environmental stress, nutritional starvation, oxygen depletion, or exposure to chemicals including antibiotics. Biofilms begin to form when cells attach to the substratum, form a monolayer then proliferate to form microcolonies and extensive networks of extracellular polymeric substances. The microcolony matures into a more complex three dimensional structure of biofilm.
As the organisms grow, problems with overcrowding and diminishing nutrition trigger shedding of the organisms to seek new locations and resources. The newly shed organisms quickly revert back to their original free-floating phase and are once again vulnerable to antibiotics. Free-floating organisms, however, in the context of site infections such as wounds, surgical sites and burns, can enter the bloodstream of a patient, creating bloodstream infections and serious infection-related consequences. Sessile rafts of biofilm may slough off and may attach to tissue surfaces, such as heart valves, causing proliferation of biofilm and serious problems, such as endocarditis.
As such, biofilms have profound health and environmental implications. Biofilms on medical devices such as catheters or implants can result in chronic infections that are resistant to therapeutic drugs. Moreover, nosocomial infections have been associated with biofilm formation on human surfaces such as teeth, skin, and urinary tract.
Biofilms are more resistant to antimicrobial agents such as antibiotics than are their planktonic counterparts. Moreover, biofilms contain persister cells that neither grow nor die in the presence of antimicrobial agents, and thus confer on them multidrug resistance. This resistance can result from the thickness of the biofilm matrix preventing penetration of antimicrobials through the biofilm, resulting in cells in the biofilm being protected from external treatment. Additionally, bacteria living in biofilms adopt an altered metabolic state, including increasing extracellular enzymatic activity inside the biofilms which confers on them more resistance to antimicrobials. Extracellular polymeric substances may form barriers or make complexes with the antimicrobials, thus preventing or reducing the antimicrobial action. Moreover, biofilms can generate different microenvironments within their layers with altered levels of CO2, oxygen, cations, pH, and other variables, which may affect the activity of antimicrobials. For these reasons, biofilms represent an important challenge for public health, necessitating the development of novel antimicrobial and therapeutic agents.